In the manufacture of semiconductor devices, liquid crystal displays and the like, patterns are formed by irradiating light onto a semiconductor wafer or a liquid crystal original plate on which resist is coated; however, if a foreign particle is attached to the photomask or the reticle (hereinafter simply referred to as a “photomask”) used on these occasions, this foreign matter absorbs light or bends light, whereby the transferred pattern becomes deformed or the edges become coarse, and the base becomes dirty black with the resulting problems of impaired dimensions, quality, appearance and the like.
These lithography operations are usually performed in a clean room, but it is still difficult to keep the photomask constantly clean; therefore, light exposure is usually carried out after placing a pellicle as a dust fender over the surface of the photomask.
Such a pellicle is generally composed of a frame-shaped pellicle frame, a pellicle film stretched over the upper end face of the pellicle frame, an airtight gasket or the like formed on the lower end face of the pellicle frame. The pellicle film therein is made of a material exhibiting a high transmittance with respect to the exposure light, and a pressure sensitive adhesive or the like is used as the airtight gasket.
If such a pellicle is placed on a photomask, the foreign matter does not directly adhere to the photomask but adheres to the pellicle for they are intercepted by the pellicle. Then, in the photolithography step, if the focal point is aligned with the pattern of the photomask, the foreign matter on the pellicle is off the focal point and hence its image is not transferred, and thus the problems such as deformation of the pattern can be suppressed.
Now, in the photolithography technology, shortening of the wavelength of the exposure light source has been progressed as means for increasing the resolution. To date, the exposure light source has shifted from g-line (436 nm) and i-line (365 nm) by mercury lamp to KrF excimer laser (248 nm) and ArF excimer laser (193 nm), and furthermore, the use of EUV (Extreme Ultra Violet) light with a dominant wavelength of 13.5 nm is also being studied.
Of late, a problem has arisen in which when a pellicle is stuck on a photomask, distortion of the pellicle causes the photomask to be distorted similarly. This distortion is referred to as PID (Pellicle Induced Distortion) here. Due to this PID, the positional accuracy of the pattern to be formed in the photolithography step is impaired, which is a big problem in semiconductor device manufacturing.
As a means for reducing the distortion of the photomask caused as the pellicle is attached to it, IP Document 1 discloses a pellicle frame having a cross-sectional area of 6 mm2 or smaller. Furthermore, IP Document 2 discloses a pellicle frame having a cross-section shaped like the letter I in order to reduce the sectional area while maintaining the width and height of the pellicle frame; likewise, IP Document 3 discloses a pellicle frame whose cross section has a plurality of hollow portions juxtaposed.